Cancer therapies that utilize the combination of small interfering RNA (siRNA) with traditional small-molecule drugs have the great potential to enhance the treatment repertoire to combat many types of cancers and are particularly important in the treatment of metastatic cancer. In order to allow clinicians to select optimal drug-siRNA combinations, an ideal delivery vector will be versatile enough to deliver siRNA and drugs ranging broadly in physicochemical properties. Importantly, mesoporous silica nanoparticles (MSN) can encapsulate a broad range of drugs in their internal porous structure and thus are well suited as the basic delivery platform onto which systems of drug-siRNA combinations can be constructed. The objective of this proposal is to design MSN capable of targeted, simultaneous, combined delivery of established chemotherapeutics and therapeutic siRNAs into metastatic cancer. The central hypothesis is that using multilayered MSN containing a redox-responsive, siRNA-binding layer and a colloidally stabilizing and targeting layer of hyaluronic acid (HA) to deliver the drug-siRNA combination into CD44-overexpressing lung metastases of breast cancer (BrCa) will improve anticancer activity as compared to either agent alone. The overall rationale for the project is that the combined drug-siRNA delivery will sensitize the cancer cells by simultaneously silencing genes involved in cancer progression while delivering effective therapeutics. The overall objective of this application will be achieved by pursuing three specific aims: 1) design, synthesize, and characterize redox-responsive multilayered MSN (RRM-MSN) for simultaneous delivery of drugs and siRNA into CD44-overexpressing cancer cells; 2) determine in vitro if combined drug-siRNA delivery using RRM-MSN improves anticancer activity in BrCa cells; and 3) determine in vivo if simultaneous drug-siRNA delivery using RRM-MSN improves antitumor activity in a lung metastasis model of CD44-overexpressing BrCa. The approach is innovative because of the versatile and modular multilayered design of nanoparticles with redox-triggered drug and siRNA release, which is suitable for the delivery of a broad range of drug-siRNA combinations. The proposed research is significant because it will establish a widely applicable and versatile method for simultaneous, targeted delivery of chemotherapeutics and therapeutic siRNAs to improve delivery and therapeutic outcome in metastatic cancer.